1. Field of the Invention
The present invention relates to a display device capable of measuring illuminance and widening a dynamic range of the measured illuminance.
2. Description of the Related Art
It is demanded to reduce an electric power consumption of an equipment having a display device. As for a liquid crystal display device, a method to reduce an electric power consumption is under study. The method is to decrease brightness of a backlight when a surrounding illuminance of a display unit is low, for example, at night. It is thought that an electric power consumption of an organic electro-luminescence display device can be reduced as well by decreasing brightness of light-emitting pixels when a surrounding illuminance is low, for example, at night. Specifically, there is a proposal to arrange a light sensor circuit in an area surrounding a display unit of such display device and use an output of the light sensor circuit to decrease a brightness of a backlight or pixels.
Liquid crystal display devices each having a light sensor circuit separated from a display panel are disclosed in Japanese Unexamined Patent Application Laid-open Nos. 1992-174819/1997-146073. However such separation makes it difficult to form devices smaller or thinner. To solve this, a technique of making a liquid crystal display device smaller or thinner by forming a light sensor circuit in a display panel is disclosed in Japanese Unexamined Patent Application Laid-open No. 2007-114315. Additionally, in the liquid crystal display device, a brightness of a backlight is controlled so as to measure an illuminance precisely.
By the way, in measuring an illuminance by the device such as the display device mentioned above, the following operation is done.
A preset count value is updated at each change of a clock signal's level. A trigger signal is outputted at time corresponding to the illuminance. The count value is sampled when the trigger signal is outputted. The sampled count value corresponds to the illuminance.
If a cycle of the change of the clock signal's level is short, a length of a period when the maximum count value becomes the minimum is short. Therefore, the count value can represent only a narrow range of an illuminance such as one from about 80 [lx] to about 1000 [lx] for example. That is, a dynamic range of an illuminance the count value can represent is as narrow as about 10 times.
Therefore, the display device can only be used in a room where an illuminance is low for example. Oppositely, the device can only be used outdoors where an illuminance is high for example. As a result, the convenience of the display device is ruined.
The present invention has been made in view of the foregoing points. An object of the present invention is to provide a display device capable of measuring an illuminance and widening a dynamic range of the measured illuminance.